The invention relates to the control of laser pointing devices, and more particularly to rectifying a pointing device to allow a pointing device with normal manufacturing tolerances to behave like a mathematically ideal device, thereby simplifying control and improving accuracy, and to improve the utility of laser pointing devices in three-dimensional applications.
Numerically-controlled galvanometers have been used in laser pointing devices for two-dimensional applications, for instance x-ray plotters, repair of semiconductor memories, and laser light shows. These generate patterns on specified two-dimensional working surfaces, e.g., x-ray film, a semiconductor chip, or a curved (but nonetheless two-dimensional) domed ceiling, respectively.
Two technical barriers have limited the use of laser pointing devices in three-dimensional applications: the computing power required to compute the trigonometry to drive the galvanometers (this barrier has fallen in recent years), and equipment and techniques to ensure that the angular control of the galvanometers is sufficiently precise and accurate while maintaining sufficient speed and acceleration.
To use a laser pointing device in a high-accuracy, high-precision application, for instance in the aerospace industry, it must be positioned very accurately over a work piece or tool if it is to illuminate points on the work piece accurately. In one known technique called resectioning, a designator (a device similar in concept to a laser light projector, though operating at higher precision, used to sweep a laser beam over a surface to illuminate a curve, for instance to indicate an outline for a work piece) automatically determines its position and orientation relative to a tool by measuring the angles to three or more fiducial points on the tool. A fiducial point, or simply fiducial, is an optical device whose position is accurately known, for instance with respect to the tool on which it is mounted. The tool is brought roughly into position with respect to the designator, for instance to within six inches. The designator (or other external optical devices) are used to sense the fiducials and measure the angles from the designator to them, and thus to accurately orient the spatial and angular position of the designator with respect to the tool.
But no matter how accurately the designator is positioned relative to the tool, the designator cannot designate points accurately if the beam deflection angles cannot be controlled accurately. Nor can resectioning be accurate if the galvanometers cannot accurately measure spatial angles to the fiducial points.
The components of the designator are subject to a number of sources of imprecision, and each designator will come off the assembly line with slightly different response to its inputs. Sources of imprecision internal to the designator include non-linearities in the galvanometer response and the position detectors, differences in gain in op-amps driving the galvanometers, bearing run-out, tolerance in the mounting of the galvanometers in the designator, twist or wobble in the galvanometer shafts, mirrors mounted slightly off-axis, variations in the mounting of the laser or other beam-steering elements, etc.
Prior approaches to compensating for errors in galvanometer-based beam positioning devices have obtained accurate beam placement on a predetermined two-dimensional surface by calibrating the beam positioning device to that surface, as exemplified in U.S. Pat. Nos. 4,918,284 and 4,941,082.
Another prior approach involves constraining certain of the designator's manufacturing tolerances as closely as possible, and precisely measuring the residual errors in the completed device. The remaining errors are measured and compensated for in the controller of the device, typically a computer, by the following method. The galvanometers are fed nominal values as if the galvanometers were ideal (nominal values are the values that should lead to a given result in an ideal device), for instance to generate angles along each of two perpendicular axes at five-degree intervals. The actual angles of the resulting beams are measured, for instance by measuring the positions of two points on each ray using a theodolite, and reconstructing the ray in space from these two points. These measurements are used to characterize the non-linearities and deviations of the physical device from the mathematically predicted behavior.